Wide bandwidth communication systems are recognized as a necessity in the transmission of video, and other high rate data signals. In particular, fiber optics communications, in general, have become increasingly popular as a means to extend the bandwidth of existing networks. Fiber optic communications will continue to exist in communication infrastructures as consumer demand for network bandwidth continues to increase. A number of different optical spread-spectrum code division multiple access (CDMA) schemes have been proposed to facilitate asynchronous multiple user access to the wide bandwidth internet and to all-optical networks. These schemes employ spread-spectrum modulation in the optical domain to divide user channels into different codes, rather than into spectral bands, that is, frequency bands, as is the case for spectrum-sliced wavelength division multiplexing (WDM) systems.
Current CDMA spread spectrum approaches have been used to provide wireless cellular phone services, and are believed to have several advantages for optical network access. A further advantage of spread spectrum systems, in general, is that spread spectrum systems are inherently secure due to the use of pseudorandom spreading code sequences that can be cryptographically varied. Spread spectrum communications also relieves network switching, thereby permitting asynchronous user access. Because each user channel is identified by a unique spreading code, data spread with a spreading code is only available to a receiver that uses the appropriate code sequence for despreading. Thus, optical spectrum spreading has application to asynchronously accessed, multiple-user, ultrawide bandwidth fiber-optic, local area networks, and secure optical communications, through both free-space and optical fibers, in wavelengths up to thousands of GHz. This is contrasted with frequency division, that is, wavelength division multiple access (WDMA) schemes where RF switching networks are necessary to channelize end user data.
In particular, optical CDMA communications is applicable to optical communications networks due to the inherent ability to increase the capacity and improve the performance of existing WDM systems, without altering the basic infrastructure of existing fiber-optic networks. This capacity increase results in significant cost savings applicable to fiber optical networks. Also, the inherent transmission security characteristics of CDMA systems makes optical CDMA an excellent means to transmit secured communications across fiber optical networks, as may be desired for application to satellite ground station networks.
Optical CDMA systems have been proposed for application to fiber-optic networking employing binary amplitude shift keying where data is susceptible to amplitude fluctuations. Optical CDMA employing pulse position-encoded spreading sequence and amplitude shift keying is susceptible to optical dispersion in optical fibers producing significant losses in matched filter detection, and as such, optical amplitude shift keying is susceptible to amplitude fluctuations resulting in degraded communications. Spectrum spreading in optical fiber systems requires noncoherent detection, because it is difficult to encode the phase of an optical signal due to fiber dispersion.
Optical CDMA approaches using bipolar codes, such as a 0 and 1 binary code transmitted as +/−1 symbols, have inherent compatibility with direct sequence multiple user CDMA systems. Unipolar coding approaches represent a 1 by signal presence and represent a 0 by signal absence such as in on/off keying of optical sources where new optical orthogonal codes are employed to optimize the correlation properties of on/off keyed systems. Optical CDMA systems may also rely on spectral encoding of an optical signature using spatial filters, such as patterned masks, or spectrally coded light sources. Unfortunately, on/off keyed systems have limited flexibility in spreading code allocation because each code is implemented in a unchangeable amplitude mask. A complementary spectral encoding approach allows for the implementation of reconfigurable bipolar codes. However, the complementary spectral encoding approach is limited in code length, and employs binary amplitude shift keyed data that is more susceptible to source fluctuations than frequency shift keyed data employed in frequency shift keying modulated dense WDM systems. Existing optical CDMA systems do not use the available optical spectrum with system flexibility and compatibility with high throughput WDM system technology. These and other disadvantages are solved or reduced using the invention.